docs: def _depression_adjustment(self, elevation):
          """Calculate the extra degrees of depression due to the increase in elevation.
  
          :param elevation: Elevation above the earth in metres
          :type  elevation: float
          """
  
          if elevation <= 0:
              return 0
  
          r = 6356900 # radius of the earth
          a1 = r
          h1 = r + elevation
          theta1 = acos(a1 / h1)
  
          a2 = r * sin(theta1)
          b2 = r - (r * cos(theta1))
          h2 = sqrt(pow(a2, 2) + pow(b2, 2))
          alpha = acos(a2 / h2)
  
          return degrees(alpha)